The present invention is related generally to methods of concentrating components in gas streams containing a mixture of components, and more particularly to a method of selectively filtering and concentrating target molecules such as toxic nerve agents in a gas stream mixture.
Semiconducting metal oxide (SMO) based sensors for chemical warfare agent detection have extreme sensitivity (parts per billion) and versatility. However, SMO based sensors exhibit poor selectivity and this is a major barrier to the implementation of this technology. In many cases, the sensor response to a target molecule is indistinguishable from the myriad of other molecules present in the gas stream. This problem of detection selectivity would clearly benefit from the development of materials and methods that can be used to selectively filter, separate, and concentrate specific target molecules from gas stream mixtures before reaching the SMO sensor.
Various methods are known for concentrating specific molecules in a gaseous mixture. For example, U.S. Pat. No. 6,171,378 to Manginell et al. discloses a chemical preconcentrator used to accumulate and concentrate one or more chemical species of interest in a gaseous mixture, and then rapidly release the concentrated chemical species upon demand for chemical analysis. The preconcentrator includes a substrate having a suspended membrane, a resistive heating element disposed on a surface of the membrane, and a sorptive material disposed on a surface of the membrane. The chemical species of interest is adsorbed on the sorptive material, and then the heating element is activated to create a thermal pulse that releases the chemical species from the sorptive material.
Unfortunately, the use of a thermal pulse to dislodge a target molecule from an adsorbent cannot achieve a high degree of selectivity between the target molecule and other similar molecules. For instance, a thermal pulse cannot effectively separate different organophosphonate compounds from each other or from other adsorbed interferent molecules. The organophosphonates include sarin and other toxic nerve agents, but the majority of organophosphonates and other adsorbed interferent molecules are harmless molecules. Consequently, a thermal pulse would not be useful for selectively filtering and concentrating the toxic nerve agents from other adsorbed interferent molecules in a gas stream mixture. Therefore, it would be desirable to provide an effective method of selectively filtering and concentrating target molecules such as toxic nerve agents.
The above object as well as others not specifically enumerated are achieved by two separate methods according to the invention for separating a target molecule from a non-target molecule in a gas stream. Preferably, the target molecule is an organophosphonate such as a nerve agent. In a particular embodiment, the method increases the relative concentration of the target molecule in the gas stream so that it can be more easily detected by a semiconducting metal oxide based sensor. In a first step of the first method, a gas stream is passed through an adsorbent. The gas stream contains molecules of the target molecule in a mixture containing molecules of at least one non-target molecule. Preferably, the adsorbent is a metal oxide such as a silica. Both the target and non-target molecules are adsorbed on the adsorbent. In a second step, another gas stream containing molecules of a chemical displacer is passed through the adsorbent. The molecules of the chemical displacer adsorb on the adsorbent to selectively displace the target molecules from the adsorbent while leaving the non-target molecules adsorbed. The chemical displacement causes the displaced target molecules to enter the gas stream. The gas stream can then be passed through a semiconducting metal oxide based sensor to detect the target molecules. Alternatively, the adsorbed chemical displacer displaces the non-target molecules while leaving the target molecules adsorbed on the adsorbent surface. In either case, a separation of the target and non-target molecules is achieved.
In the second method, an adsorbent is pretreated with a chemical displacer so that chemical displacer molecules are adsorbed on the adsorbent. A gas stream is passed through the adsorbent, the gas stream containing target molecules mixed non-target molecules. The target molecules adsorb on the adsorbent while the non-target molecules do not adsorb and instead remain in the gas stream. The chemical displacer molecules are selectively adsorbed relative to the non-target molecules to prevent their adsorption on the adsorbent. Alternatively, the chemical displacer can be selectively adsorbed relative to the target molecules, so that the non-target molecules adsorb on the adsorbent while the target molecules do not adsorb and remain in the gas stream.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.